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 impulse-reaction turbines are illustrated on pages 23 and 24.

When highly superheated steam is used the temperature is much reduced by expansion in the jets and work done in the impulse wheel before it passes to the main turbine casing.

The highest efficiency yet attained by land turbines has, however, been with the pure compound reaction type of large size, where the high pressure portion is contained in a separate casing of short length and great rigidity, now made usually of steel. The working clearances can by this arrangement be reduced to a minimum and the highest efficiency attained.

The first turbine imported into Germany in 1900, of 2000, was on this principle, while the latest turbines are of 12,000 , and generate current for the Metropolitan Railway in London.

In marine work the same principle has been almost universal since 1896, when the original single turbine of the "Turbinia" was